Chemistry

Stoked About Stoich

August 2, 2018

Stoichiometry - No Limits to Limiting Reactants

If there’s one thing virtually all chemistry teachers can agree on, it’s that stoichiometry is a difficult topic for students. A problem can involve writing chemical formulas, balancing equations, then multistep calculations converting amounts from grams to moles and back again. Just writing those sentences helps me understand why students struggle! On top of all of this, we also ask our students to identify limiting reactants and determine percent yield for an experiment.

There are a number of tools and methods teachers employ to get students through this tough topic, including flow charts, algorithms, the Before Change After (BCA) approach, and physical models to reach students. We even use analogies of bikes, cookies or hamburgers to make limiting reactants relatable.

 

Hands-on inquiry can be another practical and tangible tool. A simple experiment using household chemicals, a bottle (or flask) with a stopper and tubing, and a Wireless Pressure Sensor can give students the opportunity to easily change the amount of one reactant while quickly measuring the amount of product to see the limits of the limiting reactant.

In this experiment from our Essential Chemistry Laboratory Investigations book, students perform multiple trials, keeping the amount of baking soda (sodium bicarbonate - NaHCO3) constant while increasing the amount of citric acid (C6H8O7). To keep the procedure simple, dissolve sodium bicarbonate in water to make a 0.12 M solution. Don’t worry if you haven’t covered molarity yet - let the students know that for 1000 mL of solution, there are 10.24 g of NaHCO3. Then, when they use 40 mL of sodium bicarbonate solution for each trial, they can practice proportional reasoning to determine that there are 0.41 grams of sodium bicarbonate are in each sample.

They should mass 0.10 grams of citric acid after they add 40mL of NaHCO3 solution to the reaction vessel. After connecting the Wireless Pressure Sensor to SPARKvue and opening lab 8D in the Essential Chemistry folder, students can start data collection. Once they establish a baseline pressure they should add the citric acid and quickly stopper the bottle. Make sure one student in the group is firmly holding the stopper in place while swirling the bottle during data collection.

 

Once the reaction is complete, it’s time to analyze the data!

The change in pressure is based on the gas produced during the reaction.

Next, it’s time to repeat the experiment, but with 0.20 g of citric acid. If you ask the students to predict what will happen to the pressure most will (correctly) assume that the change in pressure will double since they have twice as much reactant. They can do the same with 0.30 g of citric acid.

Something funny starts to happen when 0.40 g of sodium bicarbonate is added. The change in pressure is not four times the 0.1 g sample. And when 0.50 grams of sodium bicarbonate is added, it is the same change as 0.40 g. How can this be?

They can graphically analyze this discrepant event this by plotting the change in pressure vs the mass of sodium bicarbonate and viewing all of 5 of the data runs.

Some students will realize that the later trials did not produce proportionally higher changes in pressure because there was not enough sodium bicarbonate to react with all of the citric acid. This is a great observation and the key to understanding limiting reactants. They have made the connection that something will run out and stop the reaction!

Based on the graphs, the third trial is closest to an ideal ratio of reactants. In trials 4 and 5, there is not a proportional increase indicating that some of the citric acid did not react. To explain this, they need to dig deeper into the data and convert masses of reactants into moles.

Looking at the third trial, they have 0.41 grams of sodium bicarbonate, and 0.30 grams of citric acid. Using the molar masses of NaHCO3 and C6H8O7, they can calculate that there are 0.0049 moles and 0.0016 moles respectively. This is a 3:1 ratio.

To put all the pieces together, one more bit of information is needed– the balanced equation!

3NaHCO3(aq) + C6H8O7(s) → Na3C6H55O7(aq) + 3H2O(l) + 3CO2(g)

There’s the reason for the 3:1 ratio of moles of sodium bicarbonate and citric acid! Anytime the reaction has something other than a 3:1 ratio of the reactants, one of the reactants limits the production of gas. Now they can then look at each of the trials, identify which reactant is limiting, and provide evidence to support their claim!

This simple experiment with household chemicals gives student the experience and data to understand the limits of a limiting reactant, how the limiting reactant can change based on the amounts of substances, and why simply adding more of a reactant does not always lead to more product. Armed with these understandings, there will be no limit to their success!

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